Aluminum is produced conventionally by the electrolysis of alumina dissolved in cryolite-based molten electrolytes at temperature up to about 950.degree. C. (the Hall-Heroult process). A Hall-Heroult reduction cell typically comprises a steel shell having an insulating lining of refractory material, which in turn has a lining of carbon which contacts the molten constituents. Conductor bars connected to the negative pole of a direct current source are embedded in the carbon cathode substrate which forms the cell bottom floor. The cathode substrate is typically an anthracite-based carbon lining made of prebaked cathode blocks which have been joined with a ramming mixture of anthracite, coke, and coal tar. A pool of molten aluminum overlying the cathode blocks acts as the cathode.
In Hall-Heroult cells, the carbon lining or cathode block material has a useful life of only three to eight years, and even less under adverse conditions. The deterioration of the cathode bottom is due to erosion and penetration of electrolyte and liquid aluminum, as well as intercalation of sodium, which cause swelling and deformation of the cathode carbon blocks and ramming mix. In addition, the penetration of air, sodium species or other components of cryolite leads to the formation of toxic compounds, including cyanides.
Difficulties in operation can arise from the accumulation of undissolved alumina sludge on the surface of the carbon cathode material lying beneath the aluminum pool, and this sludge can form insulating regions on the cell bottom. Penetration of cryolite and aluminum through the carbon body and the resulting deformation of the cathode carbon blocks can also cause displacement of the blocks, which permits aluminum to reach the steel cathode conductor bars. The aluminum will cause corrosion of the conductor bars which in turn leads to deterioration of the electrical contact, non uniformity in current distribution and an excessive iron content in the aluminum metal produced.
A significant drawback of carbon as a cathode material is that it is not wetted by aluminum. This necessitates maintaining a deep pool of aluminum (at least 100-250 mm thick) in order to ensure effective contact between the aluminum pool and the cathode blocks, and adequate protection of the carbon blocks. Electromagnetic forces, however, create waves in the molten aluminum pool, and in order to avoid short-circuiting between the cathode blocks and the anode, the anode-to-cathode distance (ACD) must be kept at a safe minimum value (usually 40 to 60 mm). For conventional cells, there is a minimum ACD below which the current efficiency drops drastically, due to short-circuiting. The electrical resistance of the electrolyte in the inter-electrode gap causes a voltage drop from 1.8 to 2.7 volts, which represents from 40 to 60 percent of the total voltage drop, and is the largest single component of the voltage drop in a given cell. Thus, it is desirable to reduce the ACD to as small as possible, while still preventing short-circuiting.
To reduce the ACD and associated voltage drop, extensive research has been carried out with Refractory Hard Metals or Refractory Hard Materials (RHM) such as TiB.sub.2 as cathode materials. The TiB.sub.2 and other RHM's are practically insoluble in aluminum, have low electrical resistance, and are wetted by aluminum. This should allow aluminum to be electrolytically deposited directly on an RHM cathode surface, and avoid the necessity for a deep aluminum pool. Numerous cell designs utilizing Refractory Hard Metals have been proposed, and these proposals should present many advantages, most notably energy savings due to the reduction of the ACD.
The use of titanium diboride and other RHM current-conducting elements in electrolytic aluminum production cells is described in U.S. Pat. Nos 2,915,442, 3,028,324, 3,215,615, 3,314,876, 3,330,756, 3,156,639, 3,274,093 and 3,400,061. Despite extensive efforts and the potential advantages of having surfaces of titanium diboride at the cell cathode bottom, however, such propositions have not been commercially adopted by the aluminum industry.
The non-acceptance of tiles and other methods of applying layers of TiB.sub.2 and other RHM materials on the surface of aluminum production cells is due not only to their cost, but also to their lack of stability under typical operating conditions. The failure of these materials is associated with the penetration of the electrolyte when not perfectly wetted by aluminum, and attack by aluminum because of impurities in the RHM structure. When RHM pieces such as tiles am employed, oxygen impurities tend to segregate along grain boundaries leading to rapid attack by aluminum metal and/or by cryolite. In fact, no cell utilizing TiB.sub.2 tiles for a cathode surface is known to have operated for long periods without loss of adhesion of the tiles and/or disintegration of the tiles. The RHM tiles have also failed due to a lack of mechanical strength and resistance to thermal shock. To combat disintegration, it has been proposed to use highly pure TiB.sub.2 powder to make materials containing less than 50 ppm oxygen. Such fabrication techniques, however, further increase the cost of the already-expensive materials.
U.S. Pat. No. 5,320,717, the contents of which is incorporated herein by way of reference, provides a method of bonding bodies of Refractory Hard Material (RHM) or other refractory composites to carbon cathodes of aluminum protection cells. The method disclosed uses a colloidal slurry comprising particulate preformed RHM in a colloidal carrier selected from colloidal alumina, colloidal yttria and colloidal ceria as a glue between the bodies and the cathode or other component. The slurry is dried to bond the bodies to the cathode or other component, and the dried slurry acts as a conductive, thermally-matched glue which provides excellent bonding of the bodies to the cathode or other cell component.
U.S. Pat No. 5,310,476 discloses a method of producing a protective refractory coating on a substrate of, inter-alia, carbonaceous materials by applying to the substrate a micropyretic reaction layer. A slurry containing particulate reactants in a colloidal carrier is applied, and a micropyretic reaction is then initiated. The micropyretic slurry optionally also contains preformed refractory material, and the micropyretic slurry may be applied on a non-reactive sublayer.
U.S. Pat. No. 5,364,513 discloses a body of carbonaceous or other material for use in corrosive environments such as oxidising media or gaseous or liquid corrosive agents at elevated temperatures. The body is coated with a protective surface coating which improves the resistance to oxidation or corrosion and which may also enhance the bodies electrical conductivity and/or its electrochemical activity. This protective coating, in particular silica-based coatings, is applied from a colloidal slurry containing particulate reactant and/or non-reactant substances, which, when heated to a sufficient temperature, form the protective coating by reaction sintering and/or sintering without reaction.
Pending application PCT/US93/05142, which is incorporated herein by reference, discloses a method for applying refractory boride coatings to cell components, particularly cathode surfaces. The boride coating are applied utilizing slurries comprising one or more coloidal carriers, and a powder additive containing pre-formed refractory borides, such as TiB.sub.2. After the slurry has been applied to the surface of the cell component, the coating is dried and then heat treated. The heat treatment improves the densification of the coating, and can even be accomplished by the increase in temperature associated with the operation of the cell itself.